The present invention relates to a methodology for detecting and predicting damage in thermal barrier coatings. Thermal Barrier Coatings (TBC's) are widely used in high temperature sections of aerospace engines and thermal power generators. More specifically, the barrier coatings are applied to turbine blades and other critical components of engines and generators to provide insulation from very high temperatures. With the application of the barrier coating, components can be used at temperatures very close to or higher than their melting point temperature.
One common method of applying thermal barrier coatings is by plasma spraying. Other methods include, for example, Physical vapor deposition (PVD) and Electron Beam Physical Vapor Deposition (EBPVD). Usually, the coating consists of a layer of partially stabilized zirconia, over an oxidation-resistant metallic bond coat of MCrAlY that is deposited on a substrate, where typically M is Ni and/or Co. During the life cycle of an engine or generator, the coatings degrade due to excess temperature and mechanical stresses. This degradation causes the TBC to peel off from the substrate thereby exposing the substrate to very high temperatures. Exposure of extreme temperatures to a TBC-free substrate could cause catastrophic engine and generator failures.
Various non-destructive evaluation methods are available for characterizing thermal barrier coatings. Conventional NDE methods include visual inspection, thermography, eddy current, microwave and ultrasonics. Optical NDE methods include laser backscatter, optical coherence tomography, mid infrared backscatter, and luminescence based piezospectroscopy. Most of these NDE techniques often detect surface condition and delamination between substrate and the barrier coating. Luminescence based piezospectroscopy detects stress changes in bond coating.
U.S. Pat. No. 6,968,730 to Schafrik et al. discloses a non-destructive method of detecting subsurface defects in thermal barrier coatings applied to gas turbine engine components. In an exemplary embodiment, the method includes positioning a evanescent microwave microscope probe adjacent a turbine component surface coated with a thermal barrier coating, and scanning the thermal barrier coating by moving at least one of the evanescent microwave microscope probe and the component surface in relation to one another in an x-y plane while maintaining a predetermined distance between the probe and the thermal barrier coating constant. In the background section of the Schafrik et al. patent, it is stated that standard non-destructive testing techniques, for example, through transmission ultrasound and X-ray diffraction may be unable to assess thermal barrier coating integrity due to density differences between the substrate and the coating. Known residual stress measurement techniques, such as X-ray diffraction, have limited use in determining thermal barrier coating quality because of the difficulty in penetrating through the thermal insulating layer to the intermediate layer.
U.S. Pat. No. 6,072,568 to Paton et al. discloses a non-destructive measurement method for determining residual stress proximate an intermediate layer in a multilayer thermal barrier coating system by directing a laser beam through an outer ceramic thermal insulating layer with the laser beam illuminating a ceramic-bearing intermediate layer in a manner to cause species present in the intermediate layer to fluoresce, measuring the frequency of the light or photons emitted by the fluroescing species, and comparing the measured frequency shift of the intermediate ceramic layer to the frequency shift determined on like ceramic material under controlled stress states to determine a representation of relative residual stress in the measured coating. The invention can be used to assess integrity or quality control of as-manufactured TBC coatings or to assess remaining coating service life of engine-run TBC coated components during an inspection or repair procedure. The background section of the Paton et al. patent states that residual stress measurement techniques such as X-ray diffraction have been of limited use in determining residual compressive stress of TBC systems due in large part to the difficulty in penetrating through the thermal insulating layer to the intermediate layer. The intermediate layer also is very thin (e.g. 1 micron thickness) and is therefore very difficult to characterize by X-ray diffraction and other conventional techniques such as neutron diffraction.
Also, U.S. Pat. No. 5,490,426 to Shiga et al. teaches a method for detecting stresses which includes the steps of dispersing a fluorescent substance in a solid portion where stresses are to be detected, measuring fluorescence decay time of the fluorescent substance dispersed in the solid portion, and detecting stresses in the solid portion based on the measured fluorescence decay time. The method enables to non-destructively detect stresses in resin-molded products without impairing their mechanical properties.
Luminescence based piezospectroscopy, described in U.S. Pat. No. 6,072,568 to Paton et al. and U.S. Pat. No. 5,490,426 to Shiga et al., is a spectroscopic technique based on laser interaction with the material. The laser based technique has limited application when the optical transparency of the TBC is reduced due to usage. Recently, spectroscopic techniques have been developed to assess the health of thermal barrier coatings. These techniques attempt to evaluate the bond coat between the substrate and the coating. Since the radiation has to pass through the barrier coating, scattering and absorption by the coating limits the applicability of the optical techniques.
Therefore, there is a need for a non-destructive evaluation technique that can provide information about the rate of degradation of thermal barrier coatings and that can provide information to detect and predict the failure of the coatings.